These yellow crystals were revealed after NASA’s Curiosity rover accidentally drove over a rock on May 30, breaking it open. Using an instrument on the rover’s arm, scientists later determined that the crystals were elemental sulfur — the first time this type of sulfur has been found on the Red Planet. Credit: NASA/JPL-Caltech/MSSS
Scientists were stunned on May 30 when a rock driven over by NASA’s Mars Curiosity rover cracked open, revealing something never before seen on the Red Planet: yellow sulfur crystals.
Since October 2023, the rover has been exploring a region of Mars rich in sulfates, a type of salt containing sulfur that forms when water evaporates. But where sulfur-bearing minerals have been found in the past—in other words, a mixture of sulfur and other materials—the rock Curiosity recently cracked open is made of elemental (pure) sulfur. It’s not clear what relationship, if any, the elemental sulfur has to other sulfur-bearing minerals in the region.
While people associate sulfur with the smell of rotten eggs (the result of hydrogen sulfide gas), elemental sulfur is odorless. It only forms in a limited set of conditions that scientists haven’t tied to the history of this location. And Curiosity found a lot of it: a whole field of bright rocks that look like the ones the rover smashed up.
NASA’s Curiosity captured this close-up image of a rock nicknamed “Snow Lake” on June 8, 2024, the 4,209th Martian day, or sol, of the mission. Nine days earlier, the rover had crushed a similar-looking rock, revealing crystalline textures — and elemental sulfur. Credit: NASA/JPL-Caltech/MSSS
“Finding a field of rocks made of pure sulfur is like finding an oasis in the desert,” said Curiosity project scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Southern California. “It shouldn’t be there, so now we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting.”
It’s one of several discoveries Curiosity has made while off-roading in the Gediz Vallis Channel, a groove that cuts through part of the 3-mile-high (5-kilometer) Mount Sharp, the base of which the rover has been climbing since 2014. Each layer of the mountain represents a different period in Mars’ history. Curiosity’s mission is to study where and when the planet’s ancient terrain might have provided the nutrients needed for microbial life, if any ever arose on Mars.
Floods and avalanches
The Gediz Vallis Channel, spotted from space years before Curiosity launched, is one of the main reasons the science team wanted to visit this part of Mars. Scientists believe the channel was carved out by flows of liquid water and debris that left behind a ridge of boulders and sediment that stretched 2 miles down the mountainside beneath the channel. The goal was to better understand how this landscape changed billions of years ago, and while recent clues have helped, there is still much to learn about the dramatic landscape.
Since Curiosity arrived at the canal earlier this year, scientists have been investigating whether ancient floodwaters or landslides formed the large piles of debris that are emerging from the canal floor here. Curiosity’s latest evidence suggests both played a role: Some of the piles were likely left by violent water and debris flows, while others appear to be the result of more localized landslides.
NASA/JPL-Caltech/MSSS NASA’s Curiosity rover captured this image of the Gediz Vallis Channel on March 31. The area was likely formed by massive floods of water and debris that piled rocks into mounds in the channel. Credit: NASA/JPL-Caltech/MSSS
These conclusions are based on the rocks found in the debris: rocks carried by water currents become rounder, like river rocks, but some debris is full of more angular rocks that may have been deposited by dry avalanches.
Eventually, water soaked into all the material that had settled here. Chemical reactions caused by the water revealed white “halo” shapes in some of the rocks. Erosion by wind and sand has revealed these halo shapes over time.
“This was not a quiet period on Mars,” said Becky Williams, a scientist at the Planetary Science Institute in Tucson, Arizona, and the deputy principal investigator of Curiosity’s Mast Camera, or Mastcam. “There was an exciting amount of activity here. We’re looking at multiple currents in the channel, including powerful flooding and currents with lots of boulders.”
While exploring the Gediz Vallis Channel in May, NASA’s Curiosity spacecraft captured this image of rocks that show a pale color near their edges. These rings, also called halos, resemble markings seen on Earth when groundwater seeps into rocks along fractures, causing chemical reactions that change the color. Credit: NASA/JPL-Caltech/MSSS
A hole in 41
All this evidence of water continues to tell a more complex story than the team’s initial expectations, and they were eager to take a rock sample from the channel to learn more. On June 18, they got their chance.
Although the sulfur rocks were too small and brittle to be sampled with the drill, a large rock nicknamed “Mammoth Lakes” was spotted nearby. Rover engineers had to search for a section of rock that would allow safe drilling and find a parking spot on the loose, sloping surface.
After Curiosity drilled the 41st hole with the powerful drill on the end of the rover’s 2-meter-long robotic arm, the six-wheeled scientist dripped the pulverized rock into instruments in the rover’s belly for further analysis. This allowed scientists to determine what materials the rocks were made of.
Curiosity has now left Mammoth Lakes and is on its way to discover what other surprises the channel has to offer.
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